The present invention relates to the manufacture of substrates. More particularly, the invention provides a technique for manufacturing a silicon-on-silicon substrate assembly. The assembly includes two substrates that are bonded together for use in the fabrication of a substrate for semiconductor integrated circuits, for example. But it will be recognized that the invention has a wider range of applicability; it can also be applied to other substrates for multi-layered integrated circuit devices, three-dimensional packaging of integrated semiconductor devices, microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d), sensors, actuators, solar cells, biological and biomedical devices, and the like.
Wafers for electronic device fabrication are often cut from an ingot, or boule, of material with an abrasive saw. The wafer often serves as both a mechanical substrate and a semiconductor material to form electronic devices in or on. One of the most common examples of this is cutting silicon wafers from a silicon ingot. The wafers are typically polished to a very fine surface finish after xe2x80x9clappingxe2x80x9d the wafer to remove the mechanical damage left by the abrasive saw, and after xe2x80x9cbacklappingxe2x80x9d the other side of the wafer to remove saw damage and to produce a wafer of the desired thickness. In some processes, devices are fabricated directly in or on the silicon wafer. In other processes, a layer of semiconductor material is grown, for example by epitaxy, on the wafer. The epitaxial layer may provide lower impurity concentrations, or be of a different semiconductor type than the wafer. The devices are formed in what is known as the xe2x80x9cactivexe2x80x9d layer, which is typically only a micron or so thick.
Epitaxial layers have been used successfully on smaller wafers and for smaller devices. Unfortunately, epitaxial layers have some associated problems that critically affect wafer yield and device yield as the size of either the wafer or the device increases. Epitaxial layers that are grown on a substrate typically adopt the crystalline structure of the substrate. In most cases, the substrate is a single crystal of a particular orientation. The most favored crystallographic orientation for growing an epitaxial layer, however, may not be the most favored crystallographic orientation for forming semiconductor devices. Additionally, surface defects or contamination on the surface of the substrate can lead to xe2x80x9cpipesxe2x80x9d, xe2x80x9cspikesxe2x80x9d, and other types of defects in the epitaxial layer. Often, a single defect will ruin a particular circuit, or cell, on a substrate. As the size of the cells gets bigger and more complex, the chance that any particular cell will fail because of a defect in the epitaxy layer increases. The size of the cells generally increases, given a particular processing technology, as the device count increases, which usually indicates an increase in circuit complexity and functionality.
The size of silicon wafers also continues to increase. Many state-of-the-art semiconductor devices are fabricated on 8-inch silicon wafers. Twelve-inch wafers are available. The semiconductor fabrication industry is moving toward using wafers of this size, but, as with most changes in technology, must solve some problems first. One of the problems is that growing a high-quality epitaxial layer on a 12-inch wafer is very difficult. Some conventional processes do not have a sufficient yield of good wafers through the epitaxial growth process to make using a 12-inch wafer economically attractive. This problem is compounded by the cost of a 12-inch substrate, which can be quite high.
From the above, it is seen that a technique for providing a substitute for an epitaxial layer that is cost effective and efficient is desirable.
According to the present invention, a technique for applying a thin film of silicon material to a target, or handle, wafer is provided. This technique separates thin films of material from a donor substrate by implanting particles,such as hydrogen ions, into the donor substrate, and then separating a thin film of material above the layer of implanted particles. The thin film can be bonded to a target wafer that provides mechanical support to form a hybrid substrate before or after separation.
In a specific embodiment, the present invention provides a process for forming a film of material from a donor substrate, typically a single crystal of silicon, using a controlled cleaving process. That process includes a step of introducing energetic particles (e.g., charged or neutral molecules, atoms, or electrons having sufficient kinetic energy) through a surface of a donor substrate to a selected depth underneath the surface, where the particles are at a relatively high concentration to define a thickness of donor substrate material (e.g., thin film of detachable material) above the selected depth.
The surface of the donor wafer is then typically attached to a target wafer, that will provide mechanical support for the thin film using a low-temperature bonding process. The target wafer can be a single crystal, polycrystalline, or amorphous, depending on the desired hybrid wafer characteristics. Energy is applied to a selected region of the donor substrate material to initiate a controlled cleaving action in the donor substrate, whereupon the cleaving action is made using a propagating cleave front(s) to free the donor material from a remaining portion of the donor substrate. The thin film is then permanently bonded to the target wafer, typically with a high-temperature annealing process.
In another embodiment, a layer of microbubbles is formed at a selected depth in the substrate. The substrate is globally heated and pressure in the bubbles eventually shatters the substrate material generally in the plane of the microbubbles, separating a thin film of silicon from the substrate.
The present invention achieves these benefits and others in the context of known process technology. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings.